vaccum advance

[Nightengale]

what exactlly does the vacum advance do, and can u run the bike without it and to what effect will that have. i am getting ready to make all the connections to my fj1100 carbs, to my 81 xs. just looking for some advice, dont want to have any problems. wish i could have used the original carbs but the were shot, i had a guy that was going to rebuild them but he said that they were shot, but he had a set of fj carbs that went to a legand car, that he sold me for the same price that he was going to rebuild my original carbs. I have read here that it has been done and is a easy switch, but i just wanted to see if anyone had any advice. i have a 81 standard with a special tank if that helps.

[crazy steve]

The vacuum advance increases timing under low or no-load operation. At higher rpms, it will help with throttle response. It was fitted for basically two reasons; one, to improve fuel economy. Two, to help reduce emissions.

You can safely run the bike without it, but expect your mileage to drop into the mid-to-low 20s. You may be able to compensate a bit by giving it more initial advance; bump the timing from 5 degrees up to 10 or even a bit more, as long as you don't get knocking/detonation that should be ok. If you have the non-adjustable timing plate used on most '81 bikes, you'll want to swap to a earlier piece.

Edit: Actually, if you're not going to run the vacuum, you could remove the vacuum can and modify the existing plate to get adjustable timing. You'll have to do a bit of fab work.... Or you might look at this:
http://www.xs11.com/forum/showthread.php?t=36055

[Nightengale]

The vacuum advance increases timing under low or no-load operation. At higher rpms, it will help with throttle response. It was fitted for basically two reasons; one, to improve fuel economy. Two, to help reduce emissions.

You can safely run the bike without it, but expect your mileage to drop into the mid-to-low 20s. You may be able to compensate a bit by giving it more initial advance; bump the timing from 5 degrees up to 10 or even a bit more, as long as you don't get knocking/detonation that should be ok. If you have the non-adjustable timing plate used on most '81 bikes, you'll want to swap to a earlier piece.

Edit: Actually, if you're not going to run the vacuum, you could remove the vacuum can and modify the existing plate to get adjustable timing. You'll have to do a bit of fab work.... Or you might look at this:
http://www.xs11.com/forum/showthread.php?t=36055

does the 81 h have a non-adjustable timing plate or adjustable?

oh and by the way steve i think that i will post all my future questions to you, since so far you have answered almost all of my questions on here. thanks again for all the help and info, being new to these bikes i need as much help as i can get. i have learned so much on here.

[ddragon63]

Ok I'll bite before I get started I will say that many may argue with me on this info... but I've logged over 200 hours in a dyno room and spent 10 years building, tuning, and driving drag cars. with that said... here is my take on timing advances.

the main thing to remember about vacuum advance is it operates under part throttle and load. it helps advance the timing durring these conditions to help get the timing advance up before the rpm is high enough for the mechanical advance to take over. Once the throttle is opened wide open there is no more vacuum load (that is if your crank case is properly vented) @ this point your motor relies on its mechanical advance to advance the timing.

I don't know if you can do it to a bike, but in performance cars we remove the vacuum advance and "curve" the distributor with lighter weights and springs untill we get the mechanical advance to operate @ the rpm of which we desire full advance. example... every hi-pro street car I've curved is 12 deg @ idle and 36 @ 2200 to 2500 rpm. Or we lock it out to full advance... this way can be hell on your starter as your trying to fire the motor @ 30 something deg's timing rather than 6 to 12.

In engine performance class (when I was in College) we learned that the only reason you really need an advance in the ignition is to aid low rpm temps, and starting. If I could lock every distributor in every motor I own @ 30 to 36 degs I would then I would never have to worry about the advance working properly.

YMMV

[crazy steve]

... here is my take on timing advances.

The main thing to remember about vacuum advance is it operates under part throttle and load. It helps advance the timing during these conditions to help get the timing advance up before the rpm is high enough for the mechanical advance to take over....

The first half of that statement is true, the second half isn't. Vacuum advance is wholly dependent on engine load, i.e. throttle opening or manifold vacuum. The bigger the throttle opening, the less manifold vacuum you have. Now, there's different ways of hooking it up that gives different results, but the most common method (and the one used on the XS) is to connect to 'manifold' vacuum to give more advance the smaller the throttle opening. On a dyno or race motor, vacuum advance is a useless item because you're operating at WOT. But on a street vehicle, it can improve fuel economy by a large factor, as well as reducing emissions at no penalty. A win-win.

'Back in the day', I owned a '67 Cougar with a 454" FE motor (.030-over 427 block/428 crank combo; with a 4-speed too! woohoo!!!). Initially installed with a single 4V intake and a de-rigeur-for-high-performance dual-point, no-vacuum-advance distributor, fuel economy was in single digits, even at cruising speeds with small throttle openings. Switching to a recurved single-point distributor with vacuum advance, mileage doubled at 60 mph freeway driving, so there's your difference. Of course, if you started mashing the gas, it quickly dropped back to single digits... But the difference was between getting 4 mpg and 10 mpg for 'average' driving, a big difference.

As to whether the '81 had an adjustable timing plate or not, a few owners have reported having one but most don't.

[ddragon63]

'Back in the day', I owned a '67 Cougar with a 454" FE motor (.030-over 427 block/428 crank combo; with a 4-speed too! woohoo!!!). Initially installed with a single 4V intake and a de-rigeur-for-high-performance dual-point, no-vacuum-advance distributor, fuel economy was in single digits, even at cruising speeds with small throttle openings. Switching to a recurved single-point distributor with vacuum advance, mileage doubled at 60 mph freeway driving, so there's your difference. Of course, if you started mashing the gas, it quickly dropped back to single digits... But the difference was between getting 4 mpg and 10 mpg for 'average' driving, a big difference.

As to whether the '81 had an adjustable timing plate or not, a few owners have reported having one but most don't.

Back then gas was cheap enough it really didn't matter... LOL

Hell I'm not very old.. When I first started racing if I bought race fuel by the barrel it cost me 4.25 a gallon... yes sir the same cost as super unleaded in most places right now. and that was in the year 2000...

Yes steve I understand what your saying. I could arguee semantics of ignition curves and air/fuel ratio all day. But I won't

Just remember your ignition can't advance past the end of its mechanical advance. the vacuum advance under load and high vacuum situations advances the timing. Most distributors (in a car anyways) with a vacuum advance, the whole mechanical advance isn't in until about 3500. I believe on an XS the mech advance isn't all in until like 5200 or something like that. SO under these rpm's, under a high vaccum load IE part throttle or stead cruise, the advance is controlled by the vacuum the engine is creating.... that's all I'm saying

[ddragon63]

and Steve??? a Dual Point?

Wow Sir that's old school!

But now I'm gonna try and be helpful

The main point I think we're missing here is that you don't want to just unplug you vacuum advance from your ignition. Your motor will run @ its preset mechanical advance untill the rpm has been reached in which the centrifugal forces overcome the weights and or springs to fully advance the motor. So in the case of an XS you'll be rolling around @ about 6 deg BTDC timing until about 5000 rpm. This will make your performance and gas mileage very poor all from the same reason. it will take more throttle to move the bike under load @ 6 deg BTDC than it does @ 20 degs+.

The spring and diaphragm in the vacuum advance is setup but the manufacture to advance the timing so many degrees base on vacuum load applied. I hope this helps!

[DAVINCI]

In engine performance class (when I was in College) we learned that the only reason you really need an advance in the ignition is to aid low rpm temps, and starting.

YMMV

You DO mean retard, right?

[ddragon63]

You DO mean retard, right?

Yes sir... My bad. Yes the only reason to RETARD the timing is for starting and control idle heat. I was getting ahead of myself. Thanks for the correction.

[crazy steve]

and Steve??? a Dual Point?

Wow Sir that's old school!

It wasn't in 1974.... 'Modern' electronic ignition was just starting to make it's appearance and was still expensive and exotic for most gearheads... CD ignitions had been around for a while, but those still used points...

[DGXSER]

The first half of that statement is true, the second half isn't. Vacuum advance is wholly dependent on engine load, i.e. throttle opening or manifold vacuum. The bigger the throttle opening, the less manifold vacuum you have. Now, there's different ways of hooking it up that gives different results, but the most common method (and the one used on the XS) is to connect to 'manifold' vacuum to give more advance the smaller the throttle opening. On a dyno or race motor, vacuum advance is a useless item because you're operating at WOT. But on a street vehicle, it can improve fuel economy by a large factor, as well as reducing emissions at no penalty. A win-win.

'Back in the day', I owned a '67 Cougar with a 454" FE motor (.030-over 427 block/428 crank combo; with a 4-speed too! woohoo!!!). Initially installed with a single 4V intake and a de-rigeur-for-high-performance dual-point, no-vacuum-advance distributor, fuel economy was in single digits, even at cruising speeds with small throttle openings. Switching to a recurved single-point distributor with vacuum advance, mileage doubled at 60 mph freeway driving, so there's your difference. Of course, if you started mashing the gas, it quickly dropped back to single digits... But the difference was between getting 4 mpg and 10 mpg for 'average' driving, a big difference.

As to whether the '81 had an adjustable timing plate or not, a few owners have reported having one but most don't.

Heck I have a fuel injected fully electronic ignition engine that I would love to see double digit fuel economy out of. 6 is bout the highest maybe 8 if your real gentle on it. But with 502 HP and max torque at 2000 RPM, who can be gentle on it! I do like watching other cars becomes dots in the rear view mirror.

[ddragon63]

It wasn't in 1974.... 'Modern' electronic ignition was just starting to make it's appearance and was still expensive and exotic for most gearheads... CD ignitions had been around for a while, but those still used points...

you would be amazed where points distributors still show up when I first bought my jet boat the po had installed a mallory marine point distributor. I would imagine this was installed just for it pure simplicity. I retro'd a pertronix mag pickup into it along with an e-core coil. He had attempted to curve his distributor. It ran funny. I locked the timing to 36 degs and she runs like a champ now.

[ddragon63]

Heck I have a fuel injected fully electronic ignition engine that I would love to see double digit fuel economy out of. 6 is bout the highest maybe 8 if your real gentle on it. But with 502 HP and max torque at 2000 RPM, who can be gentle on it! I do like watching other cars becomes dots in the rear view mirror.

With toys like that Don we calculate FPG.... Fun per gallon!

My favorite is when some who knows nothing about boats says "if you take me salmon fishing I'll put gas in the boat" then they about faint when I tell them it holds 70 gallons!

"How much gas will we use?"

Whatever it takes to get there and back lol

It's cheaper to buy salmon in the store

"Then why do you go fishing so much?"

Cuz my wife won't get up @ 5am and my boss can't swim!

[crazy steve]

Heck I have a fuel injected fully electronic ignition engine that I would love to see double digit fuel economy out of. 6 is bout the highest maybe 8 if your real gentle on it. But with 502 HP and max torque at 2000 RPM, who can be gentle on it! I do like watching other cars becomes dots in the rear view mirror.

Yeah, but you're probably running low rear gears and an auto trans with a mileage-killing high-stall convertor too, so there ya go...

I spent a lot of time working with the ignition and carburation on that Cougar to try to get the best of both worlds and met with some success. Being a stick car, I didn't have the drivetrain losses inherent in the automatics of the day, and I ran 'conservative' 3.50 gears in the rear. When it was all said and done, I could manage 16 mpg @ 65 mph on the freeway and about 10 in town if I could resist the 'urge'. But it could melt the tires at will, the trick was to drive it for acceleration, you couldn't just mash the gas. It still looked like the OEM 390GT under the hood except for the headers, so it was quite the sleeper... Did I mention I was running a 14 lb aluminum flywheel? Power could be violent and explosive...

Favorite memory of that car? A buddy dropped by with his current twit GF who started going on about 'how she loved fast cars and could I pleease give her a ride?' So I did... she started screaming as I power-shifted into third, and when we got back to my house after a very brief blast I had to dry off her seat... She got out and started screaming at my buddy that I was 'crazy' (yep, earned my nickname a long time ago) and all he said was 'I told you that's what we call him...'

[fredintoon]

what exactlly does the vacum advance do, and can u run the bike without it and to what effect will that have. i am getting ready to make all the connections to my fj1100 carbs, to my 81 xs. just looking for some advice, dont want to have any problems. wish i could have used the original carbs but the were shot, - - -

Hi Nightengale,
all this because you are missing the vacuum tap on the #2 carb?
I have a 78/79 XS11 carb rack if you want it.
Butterflys work, all 4 needle jets missing & possibly 1 or 2 needle tips bust off in the carb bodies,
all the slides are stuck full up, dunno if the diaphragms & springs are even there, ditto float bowl internals.
With all that, the body castings look OK and yes! the #2 carb has the vacuum connection.
I paid $30 for them, yours for that + postage.